1. Field of the Invention
The invention relates generally to the field of seismic surveying. More particularly, the invention relates to methods for acquiring and processing marine seismic data to determine geologic structures below the surface of the earth.
2. Background Art
Seismic surveying is known in the art for determining structures of rock formations below the earth's surface. Seismic surveying generally includes deploying an array of seismic sensors at the surface of the earth in a selected pattern, and selectively actuating a seismic energy source positioned near the seismic sensors. The energy source may be an explosive, a vibrator, or in the case of seismic surveying performed in the ocean (“marine seismic surveying”), one or more air guns or water guns.
Seismic energy which emanates from the source travels through the earth formations until it reaches an acoustic impedance boundary in the formations. Acoustic impedance boundaries typically occur where the composition and/or mechanical properties of the earth formation change. Such boundaries are referred to as “bed boundaries”. At a bed boundary some of the seismic energy is reflected back toward the earth's surface, where it may be detected by one or more of the seismic sensors deployed on the surface. Seismic signal processing known in the art has as one of a number of objectives the determination of the depths and geographic locations of bed boundaries below the earth's surface. The depth and location of the bed boundaries is inferred from the travel time of the seismic energy to the bed boundaries and back to the sensors at the surface.
Marine seismic surveying known in the art includes having a vessel tow one or more seismic energy sources, and the same or a different vessel tow one or more “streamers.” Streamers are cables having arrays of seismic sensors. Typically, a seismic vessel will tow a plurality of such streamers arranged to be separated by a selected lateral distance from each other in a pattern selected to enable relatively complete determination of the geologic structures below the sea floor in three dimensions.
The sensors used in streamers are typically hydrophones. Hydrophones are a type of sensor which generates an electrical signal or optical signal corresponding to a change in pressure. Hydrophones known in the art include a transducer, such as a piezoelectric crystal, which generates an electrical voltage when compressed. Recording equipment located on the seismic vessel is operatively connected to the hydrophones on the streamers, and makes a record with respect to time since actuation of the one or more air guns of the signal generated by each of the hydrophones.
Another type of marine seismic surveying known in the art includes positioning cables on the sea floor which include therein a plurality of seismic sensors. These cables are known in the art as ocean bottom cables (“OBC”). In seismic surveying using OBCs, a vessel on the water surface tows one or more seismic energy sources, and signals generated by the seismic sensors in the OBCs are recorded.
OBCs known in the art typically include hydrophones as seismic sensors, as do streamers towed in the water. Generally speaking, marine seismic surveys are susceptible to “ghosting” in the detected seismic signals. As is known in the art, the water surface forms an acoustic impedance boundary with the air above, and generally reflects a substantial amount of seismic energy from “upgoing” seismic waves (waves traveling upwardly toward the water surface). Ghosting is a particular problem in OBC surveys because the depth of the water is typically such that surface reflected waves are difficult to discriminate from seismic energy reflected from bed boundaries on the basis of time of arrival of the energy at the seismic sensors.
It is known in the art to include geophones or other type of sensor which is responsive to particle motion (either displacement, velocity or acceleration) in OBCs. The reason for including velocity or motion-sensitive sensors in OBCs is that these sensors are responsive not only to the magnitude of the particle motion, but also to its direction. Geophones, for example, include a wire coil suspended proximate a magnet. The coil is suspended such that it will move when the geophone is moved in response to seismic energy arriving at the geophone. A voltage is generated by the coil which is related to the velocity at which the geophone moves near the magnet. The polarity of the voltage is related to the direction that the geophone moves. It is therefore possible to determine the direction from which seismic energy arrives at the geophones. By combining geophone signals with hydrophone signals, it is thus possible to determine which parts of the detected seismic signals result from upgoing energy and which parts result from “downgoing” energy. Downgoing energy results from seismic energy reflecting off the water surface.
It is, however, relatively difficult and expensive to deploy OBCs. OBCs must be removed from the ocean floor and redeployed at new selected positions along the ocean floor in order to seismically survey a different part of the subsurface. Each time the OBCs are redeployed, the geographic positions of the seismic sensors must be accurately established in order that subsurface structures inferred from the seismic survey can be properly referenced geographically. Geographic coverage using towed streamers is much more efficient because of the relative ease with which streamers can be moved through the water, and because the streamers are positioned near the water surface their instantaneous geographic position can be determined using satellite-based systems such as global positioning system (“GPS”) receivers. It is desirable, therefore, to have a method for acquiring seismic survey data which takes advantage of the geographic coverage ability of towed streamers, with the more ghost-free images that can be obtained using OBCs.